The field of the disclosure relates generally to noise attenuation, and, more particularly, to methods and apparatus for attenuating noise in an engine nacelle.
At least some known engines, such as some known jet engines and turbofan jet engines, are surrounded by a generally barrel-shaped nacelle. At least some known nacelles include an inner barrel in which an acoustic core material provides both structural strength and noise attenuation for the nacelle. For one example, at least one known acoustic core material includes a honeycomb core, a perforated facesheet coupled to an inboard side of the core, and a backsheet coupled to an outboard side of the core. Sound waves generated inside the engine enter the cells of the core through the facesheet and reflect from the backsheet at a phase different from the entering sound waves, which tends to cancel out the incoming sound waves.
Moreover, at least some known acoustic core materials include an approximately linear material, that is, a material that responds substantially the same to acoustic waves regardless of the sound pressure (i.e., amplitude) of the waves, to facilitate noise attenuation. For example, in at least some known acoustic core materials, the facesheet and/or another layer is uniformly covered with perforations that each have a diameter on the order of a few hundredths of an inch. Typically, the perforations are arranged such that the facesheet or other approximately linear layer has a porosity of from 3 to 10 percent open area. The linearity of the facesheet increases with the density of such perforations. However, the linearity, and thus the acoustic performance, of at least some known facesheets is limited because a cost and a time required to form more of the small perforations with a necessary precision is prohibitive.
At least some known acoustic core materials include an additional linear layer to facilitate attenuation of a broader spectrum of noise. For example, at least some known core materials include a septum positioned at a partial depth within the honeycomb cells. However, to satisfy the structural strength requirements of the engine, at least some known acoustic core materials for nacelles include core cells that are “full depth,” that is, the cell walls are continuous through the core from a first surface, adjacent to the facesheet, to an opposing second surface, adjacent to the backsheet. Thus, to form a septumized core, a segment of septum material must be inserted, positioned, and secured individually within each full depth cell, which significantly increases a cost and time required to manufacture the septumized honeycomb material.
Furthermore, in at least some known acoustic core materials, the backsheet does not substantially contribute to the strength of the acoustic core material. Rather, the strength of the acoustic core material is based on the strength of the core and, additionally or alternatively, the facesheet.